When used as a substrate of thermal transfer sheet, a plastic film susceptible to heat causes problems such as deterioration in releasing and slipping efficiency and breakage of the substrate film because of adhesion (sticking) of the film to the thermal head during printing and resulting deposition of foreign matter. A method of forming a heat-resistant layer, for example, of a higher heat-resistant thermosetting resin was proposed, but the method does not improve the slipping efficiency of thermal head, although it improves the heat resistance, and demands use of a two-component coating solution because the coating solution should contain a hardening agent such as a crosslinking agent. In addition, it demands a long-term heat treatment (aging) over dozens of hours at relatively low temperature after coating for production of a sufficient hardened film, because the substrate is a thin plastic film prohibiting high-temperature processing. Thus, the method demands complicated production processes and also causes problems such as generation of cockles during heat treatment without strict temperature control and occurrence of blocking because of the contact of an opposing face with the coated face.
Addition of a lubricant such as silicone oil, low-melting point WAX, or surfactant was proposed for improvement in slipping efficiency, but use of an unsuitable lubricant causes a problem of deterioration in image intensity and image blurring because of the transfer onto the opposite face when the thermal transfer sheet is wound and the deposition of foreign matter on the thermal head during printing. Alternatively, a method of adding a filler for removal of the deposit is also known, but use of a unsuitable filler causes problems such as generation of cockles during printing by increase of friction coefficient with the thermal head and abrasion of the thermal head.
Patent Documents 1 and 2 disclose a back layer of a silicone-modified polyurethane resin; Patent Document 3, a heat resistance protective layer of a polysiloxane-polyamine block copolymer; Patent Document 4, a heat-resistant protective layer containing a silicone-modified polyimide resin, to solve the problems above, but these layers, which are lower in heat resistance as a resin, often caused problems such as sticking during high-energy printing and also problems in working environment because of use of a special solvent, demanding an additional exhaust device. Alternatively, Patent Documents 5 and 6 disclose polyamide-imide resin compositions, and Patent Document 7, a heat-resistant protective layer containing a polyamide-imide resin and a lubricant, but these materials are also insufficient in heat resistance and caused a problem of the deterioration in the quality of printed image by deposition of foreign matter on the head during high-energy printing.
As shown in FIG. 1, a thermal head used in thermal transfer recording is constituted by a heat-resistant layer 5, a heat-generating resistor 2, an electrode 3, and an abrasion-resistant layer 4 formed on a heat-releasing substrate 1, and thin-film thermal heads are commonly used. The heat-releasing substrate 1 is, for example, made of a ceramic, the heat-resistant layer 5, for example of glass, is formed as it is raised on the heat-releasing substrate 1. The maximum thickness is 20 to 150 μm, and the heat conductivity thereof is approximately 0.1 to 2 Watt/m·deg. The heat-generating resistor 2 is made of Ta2N, W, Cr, Ni—Cr, SnO2, or the like, and formed linearly by using a thin-film-forming method such as vacuum deposition, CVD, or sputtering, and the thickness thereof is approximately 0.05 to 3 μm. The electrode 3 is, for example, made of Al, and formed on the heat-generating resistor 2 for supply of electricity, in the region excluding the top area of the raised heat-resistant layer 5, and the thickness thereof is approximately 0.1 to 34 μm. The abrasion-resistant layer 4 is, for example, made of Ta2O3, SiN, or SiC.
Under the condition of thermal head, various image patterns in full color are formed and used as thermal transfer images. However among many printing conditions, in the condition where dense solid images and half-tone images are printed together i.e., when the heating energy applied to the thermal head fluctuates between high and low levels rapidly, there is caused a problem of staining due to tailing in the half-tone image, presumably by the influence of the foreign matter temporarily deposited in the area in contact between the thermal head and the back face of the thermal transfer sheet.    Patent Document 1: Japanese Patent Application Laid-Open No. Sho 61-184717    Patent Document 2: Japanese Patent Application Laid-Open No. Sho 62-220385    Patent Document 3: Japanese Patent Application Laid-Open No. Hei 5-229271    Patent Document 4: Japanese Patent Application Laid-Open No. Hei 5-229272    Patent Document 5: Japanese Patent Application Laid-Open No. Hei 8-113647    Patent Document 6: Japanese Patent Application Laid-Open No. Hei 8-244369    Patent Document 7: Japanese Patent Application Laid-Open No. Hei 10-297124